1. Field of the Invention
The present invention relates to a method and a system for estimating the location of a wireless terminal from radio signals received, and a wireless terminal therefor.
2. Description of the Background Art
Conventionally, as disclosed by Japanese patent laid-open publication No. 2004-112482, in a system for estimating the location of a radio communication terminal, base stations receive a radio signal sent from a radio communication terminal and measures the signal strength of the received signal to transmit a measurement of the strength level to an information server, which in turn uses the measurement of signal strength received to estimate areas where the radio communication terminal associated with the measurement of signal strength received can exist to determine the location of the radio communication terminal from how the estimated areas overlap with each other.
U.S. Pat. No. 6,473,038 to Patwari et al., discloses a system for locating a number of devices by measuring signals transmitted between known location devices and unknown location devices and signals transmitted between pairs of unknown location devices, entering signal measurements into a graph function that includes a number of first sub-expressions, a number of which include signal measurement prediction sub-expressions and have extreme when a predicted signal measurement is equal to an actual signal measurement, and optimizing the graph function.
Furthermore, there are some examples of location estimation of a target terminal, in which a target terminal sends signals to anchor terminals, which in turn transmit received signal strength measurements of the received signals to a location estimation server, which uses the received signal strength measurements from the anchor terminals to estimate the most probable location of the target terminal. These examples are disclosed in D. Zhao et al., “A Maximum Likelihood Estimation Method of Localization using RSSI in Wireless Sensor Networks”, Technical Report, The Institute of Electronics, Information and Communication Engineers (IECE), IN2004-327, pp. 409-414; M. Takashima et al., “An Experiment of Indoor Location Estimation using IEEE 802.15.4”, Technical Report, IECE, IN2005-7, pp. 27-32; D. Zhao et al., “A Method for Reducing Location Estimation Data Traffic in Sensor Networks”, The Society Conference Transactions in 2005, IECE, A-21-20; M. Takashima et al., “An Experiment on Indoor Location Estimation Using IEEE 802.15.4—Effect by Pedestrians and RF Sensitivities—”, The Society Conference Transactions in 2005, IECE, A-21-22; and K. Iwamoto et al., “An Iterative Location Estimation Method for Wireless Sensor Networks”, Technical Report, IECE, IN2006-39, pp. 7-14.
In the conventional location estimation systems, however, the accuracy of location estimation depends on the placing distances between anchor terminals, so that the anchor terminals must be placed densely to increase the estimation accuracy. Placement of a number of anchor terminals leads to the increase of cost of the system. Furthermore, it is conceivable that the placement of a lot of terminals is physically impossible depending on the placement circumstances. In addition, the more the anchor terminals, the higher the communications traffic for the location estimation.
In some instances, the reliability of information derived from an anchor terminal located in a certain direction is lower than that of information derived from other anchor terminals. For example, when location estimation is carried out on a person who is moving about, his/her body may be an obstacle to the estimation unless he/she carries his/her terminal overhead. Also, if there are, for instance, partitions, shelves or walls between a target terminal and anchor terminals located in a target space for the estimation, they can be obstructions to the estimation. As described, information derived from a specific anchor terminal can be less reliable, and therefore the use of such information for the location estimation may cause a decrease in estimation accuracy.
Thus, there has been a demand for a method and a system for location estimation, and a radio terminal therefor to overcome the problem of increase of communications traffic arising from the expansion of area for the location estimation and the increase of anchor terminals to be placed, thereby enabling high-accuracy estimation even if the reliability of information derived from a specific anchor terminal is lower.